1. Field of the Invention
The present invention relates to a liquid crystal display device. More specifically, the present invention relates to a liquid crystal display device including a film member such as protective film for polarizing element and retardation film.
2. Description of the Related Art
Liquid crystal display devices are widely used as a display device in various information-processing equipment such as computer and TV. And particularly in a liquid crystal TV field, there is rapidly growing demand for such liquid crystal display devices recently. For such liquid crystal display devices, further more improved display quality and reduction in manufacturing costs are strongly demanded with expansion of the market.
Under the circumstances, a liquid crystal display device in so-called vertical alignment (VA) mode has been proposed as an effective technique for improvement of display quality (For example, Japanese Kokai Publication No. Hei-11-258605, Japanese Kokai Publication No. Hei-10-153802, and Japanese Kokai Publication No. 2000-131693). In the liquid crystal display device in VA mode, liquid crystals having negative dielectric anisotropy are vertically aligned between substrates facing to each other during non-voltage application. In the liquid crystal display device in VA mode, a liquid crystal display cell hardly exhibits birefringence and rotatory polarization in the front direction. Therefore, if two polarizing elements are disposed so as to be perpendicular to each other on each side of the liquid crystal display cell, substantially perfect black display can be provided under no application of voltage, which permits very high contrast ratio. However, the liquid crystal display cell exhibits birefringence in an oblique direction. Therefore, the cell apparently has retardation and the geometric correlation of the two polarizing elements becomes not perpendicular intersection apparently, which causes light leakage, leading to reduction in contrast ratio. Accordingly, expansion of viewing angle is mentioned as a technical subject for the liquid crystal display device in VA mode. On the other hand, known is a technique in which a retardation film is provided in the liquid crystal display device in VA mode in order to cancel the retardation of the liquid crystal display cell in an oblique direction or to maintain the orthogonality of the polarizing elements. For example, Japanese Kokai publication No. Hei-11-258605, Japanese Kokai publication No. Hei-10-153802, and Japanese Kokai publication No. 2000-131693 disclose that a polarizing element is disposed on each side of a liquid crystal display cell in VA mode, and at least one retardation film is disposed between the polarizing element and the liquid crystal display cell to expand viewing angle. And a liquid crystal display device in so-called IPS (in-plane switching) mode has been proposed as an effective technique for improvement of display quality (For example, referring to Japanese Kokai publication No. Hei-06-160878, and Japanese Kokai publication No. Hei-11-305217). The liquid crystal display device in IPS mode provides display by applying transverse electric field on a horizontally aligned liquid crystal display cell disposing liquid crystal between two substrates each having a parallel aligned surface and by rotating liquid crystal molecules in an in-plane almost parallel to the substrates. The liquid crystal display device in IPS mode has advantages in that liquid crystal display cell shows birefringence hardly changing in an oblique direction and whereby the viewing angle is wide, because the device provides display by changing an angle which the liquid crystal molecules form with the polarizing element, with maintaining the liquid crystal molecules almost parallel to the substrates consistently. However, also in the liquid crystal display device in IPS mode, two polarizing elements are disposed so as to be perpendicular to each other as in the liquid crystal display device in VA mode. Therefore, the geometric correlation of the two polarizing elements is not perpendicular intersection apparently, which causes light leakage, leading to reduction in contrast ratio. Therefore, it is considered that a retardation film is provided also in the liquid crystal display device in IPS mode in order to suppress the reduction in contrast ratio. For example, Japanese Kokai Publication No. Hei-11-305217 discloses a technique in which a retardation film having controlled retardations in the in-plane direction and the thickness direction is disposed between a polarizing element and a liquid crystal display cell.
As the polarizing element in these liquid crystal display devices in liquid crystal modes, generally used is a uniaxially oriented transparent polymer film such as polyvinylalcohol resin film in which a dichromatic substance such as iodine or dichromatic dye is absorbed and oriented. However, such a polarizing element still has room for improvement in mechanical strength, heat resistance and moisture resistance. Therefore, durability of the polarizing element is generally secured by adhering a transparent protective film to both sides or one side of the polarizing element with an adhesive layer and the like therebetwen. Accordingly, the retardation film as mentioned above is generally adhered to outside of the protective film adhered to the polarizing element with a cohesive layer therebetween.
As the protective film, triacetyl cellulose film (hereinafter also referred to as “TAC”) is conventionally used widely. However, the TAC film has room for improvement in sufficient securing of moisture resistance of the polarizing element due to its high moisture permeability. Therefore, proposed is a technique in which a film such as a film made of norbornene resin more excellent in vapor impermeability than the TAC film is used as the protective film in order to further more improve durability of the polarizing element under high temperature and high humidity (For example, referring to Japanese Kokai publication No. 2004-309717, Japanese Kokai publication No. Hei-06-51117, Japanese Kokai publication No. 2002-196132, Japanese Kokai publication No. 2001-235625, Japanese Kokai publication No. 2002-221619, Japanese Kokai publication No. 2002-174729, and Japanese Kokai Publication No. Hei-11-223728). Furthermore, proposed is a technique in which a film and the like made of norbornene resin is used to give function as a retardation film for a protective film in order to reduce the number of constituting films or to improve display quality (For example, referring to Japanese Kokai Publication Hei-08-43812 and Japanese Kokai Publication Hei-08-240714). Also, a technique in which a water absorption or photoelastic coefficient of a retardation film is reduced in view of stability of birefringence (retardation) characteristics, is disclosed (For example, referring to Japanese Kokai Publication No. 2001-91744).
However, there is still room for improvement in durability of the polarizing element in order to furthermore improve display quality of a liquid crystal display device. If TAC film as a protective film is adhered to a polymerizing element formed form a polyvinylalcohol resin film, preferably used is a method in which the film is adhered to the polarizing element containing moisture and then the attached body is dried to remove the moisture, because the polyvinylalcohol resin is brittle in dry state. However, if a film having a low moisture permeability, such as a film made of norbornene resin, is used as the protective film, the moisture is difficult to dry and remove after the adhesion. Therefore, there has been room for improvement in this point.